The present invention concerns a timepiece having a case containing a mechanical watch movement driven by a spring and provided with a mechanical regulator, which is associated, via electromagnetic coupling, with an electronic regulator housed in the case, wherein:                the mechanical regulator includes a balance spring associated with a balance rotatably mounted between a plate and a balance-cock for rotation, the balance having a rim provided with at least one pair of permanent magnets whose directions of magnetisation are substantially parallel to the axis of the balance, but in opposite directions to each other; and        the electronic regulator includes at least one fixed coil arranged for cooperating with said magnets by electromagnetic coupling, a rectifier supplied by said coil and provided with at least one capacitor, and an enslaving circuit provided with an oscillator for enslaving the frequency of the mechanical regulator to the oscillator frequency by means of said electromagnetic coupling.        
The principle of a mechanical clockwork-movement powered by a spring and regulated by an electronic circuit was disclosed by J. C. Berney in U.S. Pat. No. 3,937,001. In a basic version, it is implemented by using an electric generator whose rotor meshes directly with the gear train of the mechanical movement and is thus continuously rotating. The speed of the rotor is stabilised at the appropriate rotational frequency for indicating the time, by means of an electromagnetic braking device regulated by the electronic circuit, which enslaves this frequency to that of an oscillator driven by a quartz resonator. Improvements to timepieces arranged in this manner are disclosed in U.S. Pat. Nos. 5,517,469, 5,699,322, 5,740,131, 5,751,666, 5,835,456, 6,113,259 and 6,023,446 by the same Applicant as the present Patent Application, which are incorporated here by reference insofar as they disclose the electronic circuits that can also be used with the present invention, with any adaptations required due to the fact that the electric generators are different.
The same principle forms the subject of the subsequent DE Patent Application No. 39 03 706, which schematically shows various types of electric generators that can be used in this context, including in combination with an oscillating pendulum.
FIG. 3 of the aforecited U.S. Pat. No. 3,937,001 illustrates schematically a variant which corresponds to the preamble hereinbefore, i.e. in which the rotating part of the electric generator driven by the spring of the clockwork-movement is formed by the balance of a clockwork resonator of the sprung balance type. In other words, the generator rotor of the basic version is replaced by an oscillating element, which is the balance. The latter carries two juxtaposed magnets having opposite polarities to each other, and passing opposite a fixed induction coil during oscillation of the balance. However, no construction is proposed for such a balance generator in this Patent, nor, to our knowledge, has one been made since. One particular problem, which arises in such a watch balance generator, lies in the configuration of the magnetic circuit ensuring the coupling between the fixed coil and the balance magnets, given the neighbouring metallic weights of the mechanical clockwork-movement.
A similar problem arises in electric watches of the type in which the oscillating movement of a sprung balance assembly is maintained not by a motor spring, but by electric pulses applied to at least one fixed coil arranged opposite the trajectory of the magnets, for example as is described in U.S. Pat. No. 3,487,629 and U.S. Pat. No. 3,653,199. To prevent the closed magnetic circuit passing in the plate or other metallic elements of the mechanical movement, the balance includes two parallel wheels arranged respectively on either side of the fixed coils. The magnets are arranged facing each other on the two wheels. According to U.S. Pat. No. 3,487,629, each wheel is made of a magnetically permeable material, for example soft steel, in order to close the magnetic circuit behind the two magnets that it carries. U.S. Pat. No. 3,670,492 provides another solution, consisting in using non ferrous metal balance wheels, as in conventional clockwork-movements, and adding a metal magnet support assembly behind the pair of magnets of each wheel.
The use of such a two-wheel balance in a watch of the type concerned by the present invention would be very disadvantageous, mainly because such a balance would be too cumbersome and would have too high a moment of inertia.
Indeed, the present invention aims to use as far as possible a mechanical watch movement of usual construction, simply adding an electronic regulator, which cooperates with the balance of the mechanical regulator owing to the addition of a pair of magnets on the balance. In order to do this, the only element that must necessarily be altered in the mechanical movement is the balance, because of the addition of the magnets. The natural oscillation frequency of the sprung balance assembly after alteration must be slightly higher than the original frequency, so that the electronic regulator can stabilise it by briefly braking the balance, but the frequency thus stabilised must be equal to the original frequency. It is an object of the invention to conserve, as far as possible, the other elements of the mechanism, in order to use an existing mechanical movement or similar one, for reasons of construction cost and rationalising the supply of parts.
If the conventional balance of a mechanical movement had to be replaced by a two-wheel balance in accordance with the aforecited Patents, the largest axial dimensions of the latter would require completely resizing the movement, which would become much thicker.
Another type of combination of a mechanical clockwork-movement with a regulation device by electromagnetic means forms the subject of a group of Patent Applications by Seiko Instruments Inc., particularly EP Patent Application Nos. 1 093 036 and 1 143 307, and includes a multi-polar annular magnet, mounted on the balance and cooperating with one or several fixed induction coils. These are connected by conductive wires to a switching mechanism located on the balance-cock and operating via contact with the balance spring as a function of the oscillation amplitude of the balance. This contact short-circuits the coils to brake the balance when the oscillation amplitude exceeds a predefined threshold. These coils are placed on the plate of the movement, opposite the balance rim. In a particular construction disclosed in EP Patent Application No. 1 143 307, they are grouped on a printed circuit board to form an electric circuit unit, which is installed at a location arranged for this purpose on the plate.
Since the function of such an arrangement is not to generate electric energy, but only to make the balance waste energy, no great importance is attached to the energy conversion efficiency, or to the configuration of the magnetic circuit. The presence of the coil, and other elements of the clockwork-movement in proximity to the induction coils is not inconvenient in this application, whereas it can be when, in the case of the present invention, an electronic oscillator is being powered consuming the least possible amount of mechanical energy supplied by the spring.